Keep empty Auto-Translate in-place results as no-ops

[Blackspirits]

Purpose

Current-main follow-up to the new "Translate in place" path merged in upstream commit dda1fce8f9d4a605b1b6732acc072c7109dc6323 (SubtitleEdit#14956 follow-ups).

The existing selected-lines flow already treated an OK result containing no translated rows as a no-op when it was about to capture the current subtitle as the original. The new in-place mode accidentally made that guard conditional on captureOriginal.

Current upstream main was rechecked at dda1fce8f9d4a605b1b6732acc072c7109dc6323, exactly this PR's base.

Finding fixed

For a subtitle without an original column:

1. choose Translate in place;
2. let the Auto-Translate dialog return OK but with every TranslatedText empty (for example a partial/cancelled run that produced no row);
3. no subtitle Text is changed;
4. nevertheless AutoTranslateSelectedLines() continued into OnSubtitleLanguageChanged().

OnSubtitleLanguageChanged() is intentionally stateful: it clears the cached detected language, clears the dictionary explicitly selected for the current subtitle, drops an in-progress spell-check session and re-runs live-spell-check language selection.

That is correct after translated text replaces rows, but not when the operation produced zero changes.

The pre-existing default/capture-original mode did return early for this exact empty-result case; the new in-place branch bypassed that no-op guarantee.

Fix

After OK, selected-lines Auto-Translate now first checks whether any result row contains non-empty TranslatedText.

If none do, it returns immediately before:

• copying rows into OriginalText;
• changing any subtitle text;
• clearing detected-language/spell-check state;
• switching translator mode;
• refreshing the audio visualizer.

Only after a real translation exists does the method decide whether this run should capture the original or remain in-place.

Regression coverage

A headless UI regression:

• creates a normal subtitle with no original column;
• seeds the private detected-language cache with a sentinel;
• opens selected-lines Auto-Translate with Translate in place enabled;
• returns OK with every translated row empty;
• asserts text and original columns remain unchanged;
• asserts translator mode remains off;
• asserts the detected-language cache remains untouched.

The existing positive in-place translation and default capture-original tests remain unchanged.

Scope / branch state

• base/current upstream main: dda1fce8f9d4a605b1b6732acc072c7109dc6323
• HEAD: 95d3546056de205091f9fa4fd1475f01399a4f36
• 1 commit
• 2 files
• +47 / -6
• independent of Map saved Auto-Translate languages across providers #154
• audit draft only; no merge/promotion intended

Acceptance

• full restore/build/test suite on this exact HEAD;
• no new warning attributable to this tranche;
• retry step unused;
• final adversarial review must confirm zero-result behavior is a true no-op in both in-place and capture-original modes;
• upstream drift rechecked before freeze.

AI assistance: ChatGPT was used for state-transition review of the new in-place path, spell-check/language side-effect tracing, and focused no-op regression design.

Final CI

Authoritative run: SubtitleEdit#313 on 95d3546056de205091f9fa4fd1475f01399a4f36

• SeConv: 505 passed / 2 skipped / 0 failed
• LibUiLogic: 918 passed / 0 skipped / 0 failed
• LibSE: 2030 passed / 0 skipped / 0 failed
• UI: 5411 passed / 10 skipped / 0 failed
• build: 0 errors / 8 pre-existing warnings
• retry step skipped
• failure-artifact upload skipped

All eight warnings are outside the two-file #155 changed set.

Final drift gate

Upstream main was rechecked at dda1fce8f9d4a605b1b6732acc072c7109dc6323, exactly this PR's base. No post-base drift exists.

Final adversarial review was performed on this exact HEAD after CI; no blocker found.

[Blackspirits]

Final adversarial review on 95d3546056de205091f9fa4fd1475f01399a4f36 after CI SubtitleEdit#313: no blocker found. Verified zero-result OK is now a true no-op before original capture, row mutation, detected-language reset, dictionary/spell-check state reset, translator-mode switch and waveform refresh. Positive in-place and capture-original paths remain unchanged. All eight warnings are outside the changed set; upstream is still exactly dda1fce8.... No merge/promotion performed.